1. Field of the Invention
This invention relates to differential voltage measurement circuits, and more specifically to measurement circuits and methods for monitoring the voltage of a number of battery cells.
2. Description of the Related Art
A battery is typically comprised of a number of individual cells that are connected in series to produce an overall battery voltage. Some battery types, such as nickel-cadmium or nickel-hydrogen, are rechargeable. When maintaining such a battery, several problems may occur that irreversibly damage the battery or reduce its output or lifespan. For example, overcharging the battery can cause fluids within the individual cells to boil away, reducing their life and output capability.
One procedure commonly performed on rechargeable batteries is called "reconditioning." When reconditioning a rechargeable battery, each of the individual cells is intentionally discharged. While discharging, it is important to monitor the cell voltage. When any one of the cells is almost completely discharged, the discharging procedure must be stopped to prevent the voltage across the drained cell from reversing and irreversibly damaging the cell. Such a cell no longer contributes to the output of the battery and can act as a resistor among the remaining cells, thus reducing their performance.
To avoid these problems, it is advantageous to monitor the voltage across each battery cell as the battery is being recharged or discharged. This cell voltage monitoring has problems of its own, however. With a 60 cell battery, for example, with each cell producing a maximum voltage of 1.6 volts, total battery output is a maximum of 96 volts. Since the voltage of each individual cell must be monitored, a differential voltage measurement is called for. In this example, the voltage on the anode (positive terminal) of the cell at the top of the stack will be at 96 volts, and the voltage on the cathode (negative terminal) of that cell will be at 96-1.6=94.4 volts. This differential voltage cannot be measured directly with a conventional differential amplifier because both inputs exceed present amplifiers' maximum allowable input voltage specification, and the common-mode voltage (94.4 volts) exceeds the maximum common-mode voltage specification. These specifications are typically expressed in relation to the amplifier's supply voltage. Assume a differential amplifier receives a supply voltage of .+-.18 volts, a typical maximum. The amplifier's maximum allowable input voltage will be equal to about .+-.16.5 volts, and the maximum allowable common-mode voltage will be about .+-.12 volts. With one input at 94.4 volts and one at 96 volts, both the maximum allowable input voltage and the common-mode voltage specifications are exceeded.
One circuit that has been employed to independently measure 32 cell voltages is shown in FIG. 1. A group of eight series-connected cells 10 are connected to a voltage-to-current converter A1 (implemented with an operational amplifier), through a set of analog switches 14, such that converter A1 is connected across one cell at a time, and sequences through each of the cells 1-8. The converter A1 transforms the differential cell voltage into a ground-referenced current which is proportional to the cell voltage. The eight cells 10 each produce a maximum voltage of 1.6 volts, so that the cathode 16 of CELL 1 is at ground potential and the anode 18 of CELL 8 is at 12.8 volts. The converter A1 receives power supply voltages +PS1 and -PS1 from a floating power supply PS1 needed to provide adequate input voltage and common-mode voltage ranges. To measure each of the eight cell voltages independently, pairs of switches 14 are sequentially closed so that each cell is connected to the converter A1 in turn.
Similarly, the next series 20 of eight cells and their corresponding analog switches 22 are connected to another voltage to current converter A2, which must accommodate input voltages ranging from 12.8 to 25.6 volts. The voltage to current converter A2 receives power supply voltages +PS2 and -PS2 from a floating power supply PS2 for this range of input voltages. The remaining cells are handled the same way, with additional converters A3 and A4 requiring separate floating power supplies PS3 and PS4, respectively. The outputs of the four converters A1, A2, A3, and A4 are multiplexed together and fed to an output amplifier A5, which converts the ground-referenced current signal produced by each converter A1-A4 to a ground-referenced voltage.
This circuit, requiring four floating power supplies and four voltage to current converters, is costly and complex. The output transistors used by converters A1-A4 become electrically stressed for battery voltages greater than about 60 volts. This limitation renders the circuit impractical for batteries producing higher voltages.
Multiple cell rechargeable batteries have become increasingly prevalent in applications such as satellites and electric vehicles. Maximum voltage requirements are increasing for these and similar applications, and recharging parameters are becoming stricter as the demand for more powerful and longer-lasting batteries increases. A strong need exists for a way to accurately monitor individual battery cell voltages, and more generally, for a way to measure a number of differential voltages when one or more measurement points are at a high voltage.